This invention relates generally to axle drives for motor vehicles and, more particularly, to a transaxle drive differential assembly.
As is known, the use of a differential assembly in the transmission system of a motor vehicle allows the wheels to spin at different speeds. In the case of a vehicle traveling in a straight line, the axle shafts connected to the differential assembly will rotate at the same speed. However, when a turn or curve is encountered, the axle shaft nearest the inside of the turn will slow in rotational speed while the outer axle shaft will simultaneously increase in rotational speed. As such, the wheels, driven by the axle shafts, are prevented from scuffing the surface across which they travel.
An example of a transaxle drive differential assembly for motor vehicles may be seen in U.S. Pat. No. 4,232,569, issued on Nov. 11, 1980. In the differential assembly disclosed in the '569 patent, the ring gear which carries the differential gearing is configured with a reduced outer diameter adjacent to the gear teeth. This diameter forms a stepped surface which is placed in contact with a mating surface formed in the housing which surface acts as a plain bearing surface for the differential assembly. The principal advantage of this configuration is the compact configuration provided to the differential assembly and the use of the housing to take up the gear separating forces instead of the axle shafts.
While the differential assembly disclosed in the '569 patent works well for its intended purposes, this configuration does suffer disadvantages. In particular, the fabrication of the ring gear is relatively expensive in that the step adjacent to the gear teeth requires a larger part size which is costlier when compared to comparable strength differential ring gears. The cost is attributable to the additional material, the requirements of handling a larger part, and the larger manufacturing machine size required. Even in a configuration where the part can be potentially formed in a net shape via sintered powder metal, the stepped gear configuration is not desirable due to stresses caused at the transition between the larger and smaller diameters and due to variations in the densities of the sintered metal.
Additionally, in the '569 patent, the differential configuration disclosed is used in a housing with a horizontal split line. This split line configuration requires relatively high tolerances between upper and lower housing sections to reduce discontinuities at the split line. Any such discontinuities between the upper and lower housing sections create a step that can cause uneven loading of forces between the housing sections as well as binding of the differential and increased noise.
A further differential assembly design is disclosed in U.S. Pat. No. 4,480,501 issued on Nov. 6, 1984. In the configuration disclosed in the '501 patent the tips of the gear teeth of the ring gear are used as a plain bearing surface in contact with a bearing shoe that forms a mating plain bearing surface. This arrangement eliminates the problems caused by a discontinuities between housing halves and allows the use of a narrower ring gear since the need for a step is eliminated. However, this arrangement suffers the disadvantage of requiring the gear teeth to be used as plain bearings which causes additional wear of the gear teeth and the potential for greater than usual contamination of the gear box assembly. A further disadvantage is the requirement of fabricating and retaining the bearing shoe and the associated additional cost and complexity of manufacture. Yet another disadvantage associated with both the '569 and '501 patents is the need to configure the ring gear to carry the bevel gears, or to increase the size of the bevel gears in order to match the internal diameter of the ring gear. As such, the gear sizes are optimized not for transmitted torque but for the space required to fit the bevel gears.
From the foregoing, it is clear that there is a unfulfilled need in the art for an improved transaxle drive differential assembly which is free of the disadvantages above-described. Accordingly, it is an object of the present invention to provide a transaxle drive differential assembly having improved cooperable bearing surfaces whereby the need to provide a bearing shoe or ring gear with a stepped surface is eliminated. It is a further object of the present invention to provide a differential assembly having a reduced number of fasteners thereby making the differential assembly easier to manufacture.